1. Field of the Invention
The present invention relates in general to a fluid-filled vibration damping device having an electromagnetic actuator adapted to oscillate or displace an oscillating member which partially defines a fluid chamber formed in the vibration damping device, for controlling the pressure of a fluid in the fluid chamber, in order to actively damp vibration to be damped. More particularly, this invention is concerned with such a fluid-filled active vibration damping device which is capable of oscillating or displacing the oscillating member with high stability and efficiency and which is suitably used as an engine mount, a body mount or a cab mount for a motor vehicle, for example.
2. Discussion of the Related Art
As one type of a vibration damper or a vibration damping connector which is interposed between a vibration source and an object whose vibration is to be damped, there is known a fluid-filled active vibration damping device including: (a) a first and a second mounting member which are spaced apart from each other; (b) an elastic body which elastically connects the first and second mounting members and which partially defines a fluid chamber filled with a non-compressible fluid; (c) an oscillating member which partially defines the fluid chamber; (d) an actuator for oscillating the oscillating member; (e) a flexible diaphragm which partially defines an equilibrium chamber filled with the non-compressible fluid; and (f) means for defining an orifice passage for fluid communication between the equilibrium chamber and the fluid chamber.
In such an active vibration damping device, the oscillation or displacement of the oscillating member is controlled on the basis of characteristics of vibration, such as its frequency or amplitude, for thereby controlling the pressure of the fluid in the fluid chamber, so that the vibration is actively or positively damped. It is proposed to employ such an active vibration damping device as an engine mount for a motor vehicle, in view of its capability to provide a sufficiently high vibration damping effect with respect to vibration whose frequency and amplitude are variable.
For such an active vibration damping device to be able to stably and effectively provide a desired vibration damping effect, it is necessary that the oscillation of the oscillating member is stably and efficiently controlled. In this respect, there has been proposed an active vibration damping device as disclosed in JP-A-59-1829 and JP-A-61-2939, which has an electromagnetic actuator adapted to oscillate or displace the oscillating member. In this active vibration damping device, the oscillating member is driven by an electromagnetic force, i.e. a magnetic force generated by an electromagnet, so that the oscillation of the oscillating member is easily and accurately controlled.
However, even such an active vibration damping device equipped with the electromagnetic actuator suffers from deterioration of stability of the displacement of the oscillating member, since the oscillating member, upon displacement thereof, tends to be inclined by, for example, a force other than the electromagnetic force applied thereto by one or more other element or member of the vibration damping device, resulting in difficulty in providing a desired vibration damping effect, with high stability, irrespective of operating conditions. For solving this drawback, it is considered possible to use a guide mechanism in which the oscillating member is slidably guided by a slideway or slide bushing in an oscillation direction in which the oscillating member is displaced or oscillated, so as to prevent or minimize inclination of the oscillating member with respect to the oscillation direction. However, the sliding movement of the oscillating member on the slideway or in the slide bushing tends to generate an undesirable noise and heat due to a frictional resistance between the oscillating member and the slideway or bushing, and even deteriorate the oscillation motion of the oscillating member due to an energy loss. In this respect, the use of the electromagnetic actuator as means for oscillating the oscillating member still has a technical problem to be solved.